U.S. Pat. No. 5,142,162 discloses a control system for the electrical system of a vehicle, the battery being separated from an electrical load in order to avoid an exhaustive discharge of the battery. For this purpose, the battery is connected to a transistor which, as the switch, either connects parasitic loads to the battery or separates them from the battery.
EP 0 542 365 A1 discloses a circuit in which a battery is protected by a field-effect transistor (MOS-FET) against an exhaustive discharge. According to EP 0 542 365 A1, the field-effect transistor enables the load to be supplied by the battery for as long until the battery voltage of the battery drops below a specified voltage value (gate-to-source cut) of the field-effect transistor.
US 2005/0237686 A1 relates to a method for avoiding exhaustive discharges of a secondary battery. According to this method, a switching element is connected in series with the secondary battery. If the battery is discharged below a predetermined voltage level, the switching element separates the secondary battery from the load circuit. According to this solution, a Schmitt trigger, in particular, is used as switching element.
WO 2010/080017 A1 relates to an electronic protection circuit which prevents an exhaustive discharge of car batteries.
In general, traction batteries for use in electric drives of vehicles are constructed in accordance with the current state of the art as described in the text which follows: a small unit forms the actual energy cell. This comprises, for example, components with Li (Nix, COy, Mn2) O2 mixed with LiMn2O4 or LiFePO4 as active materials. As a consequence of its chemical-physical nature, such a battery cell has a voltage range which lies between 2.8 volts-4.2 volts. If such a battery cell is fully charged, which corresponds to an SOC of 100% (SOC=State of charge [Ah] divided by the maximum capacity possible according to the current service life of the cell [Ah times 100%]), the open circuit voltage of such a battery cell is usually 4.2 volts.
Such battery cells should not be operated outside their voltage range since otherwise the battery cell can be damaged with partially safety-critical consequences. The battery cells which are installed in accordance with the state of the art have—as described above—a voltage range between, for example 2.8 volts-4.2 volts. At lower temperatures, thus for example, at temperatures around about −25° C., the lower limit of operation can be lowered to 2.1 volts in most cases due to the higher internal impedance of the battery cell in order to provide for a voltage dip when performing a cold start, for example of a hybrid vehicle.
In general, accumulator cells (battery cells) have a typical open terminal voltage characteristic (OCVOpen Circuit Voltage) in dependence on the dispersed capacity between positive cathode and negative anode.
In traction batteries as are used for electric drives of vehicles, the battery cells can usually be operated within a particular charge window. A charge window is given, for example, by an SOC window, with SOC of 90% down to a lower level of a state of charge SOC of 10%. Corresponding state-of-charge windows, i.e. SOC windows, are adapted to the respective all chemistry of a battery cell and provide for an acceptable aging characteristic. However, it has been found that the further the state-of-charge limits (SOC limits) are extended, the worse an aging characteristic is obtained with a battery cell in operation. In the case where battery cells are discharged further than the lower limit of the state of charge SOC of 10% SOC, the battery cell voltage breaks down very rapidly to very low values. If the voltage of a battery cell has dropped below such a state of charge, this is an exhaustively discharged battery cell. In this case, the corresponding battery cell can be damaged reversibly or irreversibly depending on the chemical construction. In this case, a normal re-use of such a battery cell represents a safety-critical state.
In this case, repair of a corresponding battery pack is required which, as a rule, is associated with an exchange of the battery cells affected. A correspondingly affected electric vehicle or hybrid vehicle must then be classified as a broken down vehicle. This means that an exhaustive discharge of individual battery cells installed within a battery pack should be avoided if possible since great costs are caused and a not inconsiderable damage to the image could arise.